1. Field of the Invention
This invention relates to optics and more particularly to coupling electromagnetic energy between a fiber and a substrate.
2. Description of the Prior Art
In an electrical communication system, a message comprises an electrical signal transmitted through a copper wire which spans a distance between two terminals. The wire is an undesirable feature of the electrical system because copper is expensive, heavy and (because of extensive usage) will ultimately be in short supply. Since the wire is undesirable, an attractive alternative to the electrical system is an optical communication system where a light-conducting fiber spans the distance between the terminals and a message comprises a modulated light beam transmitted therethrough.
In addition to the fiber, the optical system may include either a channel or a planar waveguide comprised of what is known as an electro-optic material. The electro-optic material has a high refractive index that changes in response to a change of a voltage applied thereto. Being formed of the electro-optic material, the waveguide may be used as a signal processing element, such as a modulator, a demodulator, or a switch.
An essential feature of such an optical system is a coupling of light into the fiber from the waveguide, and vice versa. The light may be coupled by a type of optical coupler predicated upon evanescent waves emanating from a conductor of electromagnetic energy.
The existence of evanescent waves is the subject matter of a well known experiment where a beam of light is transmitted along a line of entry to a transparent first prism. The first prism has two faces at right angles to each other, and a hypotenuse face which forms a forty-five degree angle with each of the right-angle faces.
The line of entry is perpendicular to one of the right-angle faces whereby the hypotenuse face has the beam transmitted thereto with an angle of incidence of forty-five degrees. The first prism has a refractive index that causes a total reflection of the beam within the first prism from the hypotenuse face thereby causing the beam to exit from the first prism perpendicularly through the other right angle face.
A second prism, similar to the first prism, has a hypotenuse face which is placed against the hypotenuse face of the first prism, whereby the prisms form a transparent rectangular solid. The beam is transmitted through the rectangular solid along the line of entry. However, when the hypotenuse faces are separated by a small distance, typically less than or equal to a wavelength of the beam, one portion of the beam is reflected from the hypotenuse face of the first prism, as described hereinbefore, while another portion of the beam is transmitted through the prisms along the line of entry.
This experiment demonstrates that the reflection of the beam from the hypotenuse face of the first prism causes an existence external thereto of waves of electromagnetic energy. The waves of electromagnetic energy are the evanescent waves referred to hereinbefore. In the optical coupler, the coupling into the fiber from the waveguide (and vice versa) is via evanescent waves.
The first prism is referred to as a distributed optical coupler because coupling is provided over a length that is many times the wavelength of the coupled electromagnetic energy (i.e., the length of the hypotenuse face of the first prism). A distributed coupler causes electromagnetic energy transmitted thereto from an input device, such as a fiber, to have electric and magnetic moments which vary in space and in time synchronously with the phase of a wave of electromagnetic energy which could propagate in a coupled output device, such as a waveguide. The synchronous propagation of electromagnetic energy is referred to as phase matching.
Phase matching may be achieved when a distributed coupler is a periodic diffraction grating disposed in a coupling region where the grating couples the waveguide to the fiber via evanescent waves. It should be understood that the grating, unlike the prisms, is substantially planar. Additionally, the waveguide and the fiber are substantially planar. Therefore, the grating, the waveguide, and a coupled portion of the fiber may be integrated into a planar assembly that is mechanically stable and durable.
Usually, the waveguide has a thickness on the order of one micron. Because of the one micron thickness, the waveguide is mounted on a supporting structure known as a substrate. Typically, the distributed grating coupler has associated with it a problem of inefficiency because the grating couples a portion of the electromagnetic energy (light) into the substrate.